Breast cancer screening has been recommended for many decades, particularly in women over the age of fifty. The combination of early detection and improved therapy in the U.S. has resulted in a significant reduction in breast cancer mortality, with similar reductions being observed in other countries. Despite the success of screening mammography, however, it is also recognized that mammography is a less than perfect screening method. The limitations of mammography are particularly evident in women with mammographically dense breasts. It has been shown that the sensitivity of mammography decreases with increasing mammographic density, and is less than fifty percent for women with an extremely dense breast pattern on a mammogram.
The reduced sensitivity of mammography with increasing mammographic density is compounded by the fact that increased density is a significant risk factor for breast cancer. Given that a dense breast pattern occurs more frequently in younger women, this factor significantly diminishes the value of mammography in the screening of young women who have a high familial risk of breast cancer.
A second major limitation to screening mammography is in the evaluation of women at high risk of breast cancer. Numerous studies have demonstrated that in women with a high genetic risk of breast cancer, mammography has a sensitivity of between 33-43 percent. Most of these studies have been performed in women with an average age of forty, so part of the explanation for the poor performance of mammography in these studies may be due to the presence of dense breast patterns in a significant percentage of the mammographic images.
A possible solution to the problem of the detection of breast lesions in dense breast tissue is to use ultrasound in such patients. Ultrasound is attractive for supplemental screening because it is widely available, is well-tolerated by patients, and involves no radiation. However, while supplemental ultrasound screening uncovers more breast cancers, it also substantially increases the risk of a false positive cancer finding and unnecessary biopsy. Hence, the use of whole-breast ultrasound as a sole identifier of breast malignancies is questionable. Even in combination with mammography, the two anatomical techniques have significant limitations. It would be of considerable benefit to provide a complementary method that provides functional information about lesions seen on ultrasound. Such a method would significantly reduce the number of false positive cases, and allow the radiologist to evaluate those lesions that demonstrate both a functional and anatomical abnormality.
Over the last five years, several nuclear medicine-based technologies have been developed that have application in breast imaging. Included in these are positron emission mammography (“PEM”) and molecular breast imaging (“MBI”). In PEM the breast is compressed between two opposing detectors and the 511 keV gamma rays emitted by a positron emitting radiopharmaceutical, such as F-18 fluoro-deoxyglucose, are detected by coincidence imaging between the two opposing detectors. The PEM images provide an image of glucose utilization by breast tissue and have been shown to be capable of detecting small cancers in the breast. Unlike anatomical techniques such as mammography and ultrasound, PEM is not influenced by dense breast tissue.
The second nuclear medicine-based technique is MBI. This technology employs one or two small gamma cameras. The breast is compressed between a camera and a compression paddle, or between two gamma cameras, and radiation emitted by single-photon radiopharmaceuticals, such as Tc-99m sestamibi, is detected after collimation. MBI is a planar imaging technique without tomographic capability; however, information from two opposing gamma cameras can be used to calculate the true depth of a functional abnormality in the MBI images. The MBI system has been shown to have a very high sensitivity, for example in some cases greater than ninety percent, for the detection of lesions smaller than ten millimeters. In addition, it has been found that, in some cases, MBI can detect three times as many cancers as digital and analog mammography in asymptomatic women at increased risk of breast cancer.
Beyond sensitivity differences, technologies that provide functional images of the breast, such as MBI, can detect lesions not visible with conventional mammography. Likewise, certain benign breast conditions may result in a false positive finding on MBI, and this uptake can be readily identified as a benign process from the anatomical information available in ultrasound. Currently it is not practical to fuse anatomical images from ultrasound systems and functional images from MBI. Ultrasound requires that the patient lie supine and a handheld scanner is then used to scan the breast. MBI is usually performed with the patient seated and the breast lightly compressed between the gamma cameras or camera and paddle. MBI employs light compression forces, for example 10-15 pounds of force, with imaging times in the 5-10 minute range. The imaging procedure is generally considered to be substantially pain-free. Because of the differences in patient orientation between MBI and ultrasound, the shape of the breast tissue is significantly different between the two modalities and, hence, correlation of an anatomical abnormality with a functional abnormality becomes difficult. Therefore, accurate co-registration of anatomical images from ultrasound and functional information from MBI is not currently possible.
Over the last few years, several entities have worked on the development of whole-breast ultrasound (“WUS”) systems. The main purpose of this development was to reduce the dependence of image quality on the technologist or radiologist, and provide a more reproducible imaging technique. These systems are designed to image the patient in the supine position in a comparable manner to conventional breast ultrasound. If the patient is not supine, then the WUS system suffers from a loss in the achievable coverage of the breast tissue. Therefore, while WUS systems provide better coverage in non-supine patient positions than traditional ultrasound imaging, they are still limited in their applicability to combination with imaging modalities that require non-supine patient positions, such as MBI.
It would therefore be desirable to provide an MBI system that would allow the acquisition of both anatomical and functional images of the breast, such images being amenable to co-registration so that accurate and reliable assessments of the presence of cancerous lesions in the breast can be made. Additionally, it would be desirable to provide an MBI system that would also allow for breast biopsies to be performed under the guidance of MBI.